Duplex stainless steel is stainless steel having Cr, Ni, and No as main elements and adjusting the ferrite and austenite to a phase ratio of about 50% so as to secure toughness and corrosion resistance. When welding this duplex stainless steel, most of the time, from the viewpoint of maintaining the corrosion resistance, no heat treatment is applied after welding. The weld metal is used as solidified. Therefore, compared with steel materials of the same composition which are rolled and heat treated, the weld metal becomes remarkably coarse in crystal grain size and inferior in toughness and ductility. Therefore, when welding duplex stainless steel, refining the solidified crystal grains of the weld metal could be an effective method for improving the toughness and ductility of the weld metal.
As methods for refining the crystal grains of stainless steel, the method of defining the rolling conditions of the steel slab (relation of reduction rate and temperature) for suppressing the occurrence of roping (unevenness of surface) (for example, see PLT 1) and the method of defining the hot rolling and cooling conditions after casting (for example, see PLT 2) have been disclosed, but both of these utilize control of the structure by transformation in the reheating-hot rolling or annealing-cooling process after solidification of the molten steel. They are not techniques for refining the crystal grains in the process of solidification of weld metal. They are not methods effective for refining the grain size of the weld metal of duplex stainless steel enabling use as solidified after welding.
As methods of refining the crystal grains of stainless steel weld metal as solidified, methods of using inclusions as seeding nuclei for causing solidification of equiaxial crystal grains are being disclosed (see PLT's 3 and 4). These are martensitic stainless steel and austenitic stainless steel and differ from the duplex stainless steel covered by the present invention in the ratio of the amount of Mn and the amounts of Cr/Ni.
Further, in duplex stainless steel, due to the recent soaring prices of Ni and Mo, inexpensive type duplex stainless steel reduced in amounts of Ni and Mo (for example, see PLT 5) is being developed, but even when welding such duplex stainless steel, duplex stainless steel-based welding materials with coarse solidified crystal grains like in the past are being used.
In view of this backdrop, development of wire for welding duplex stainless steel able to be used even for inexpensive type duplex stainless steel, enabling the solidified crystal grains of the weld metal to be refined, and as a result enabling a weld zone with good toughness, ductility, and other mechanical properties of the weld metal to be obtained even in the state as welded has been desired.